FIG. 1 shows a general conventional hydraulic apparatus in a fork lift truck. In this hydraulic apparatus, an engine 2 drives a hydraulic pump 4 to feed under pressure hydraulic fluid in an oil tank to a control valve unit 8 and to tilt loading/unloading control levers 10 and 12 of the control valve unit 8, thereby supplying or discharging a necessary quantity of hydraulic fluid to or from a lift cylinder 14 or a tilt cylinder 16. A fixed flow type flow divider (not shown) is provided in the control valve unit 8, and a driving control system circuit 18 comprising a brake booster 20, a clutch booster 22 and a power steering gear box 24, etc. is connected to the fixed flow outlet of the fixed flow type flow divider.
Heretofore, since the hydraulic pump 4 has been a fixed displacement type, the discharge quantity Q.sub.0 of the hydraulic pump 4 is always constant if the rpm of the engine 2 is constant. Since the flow divider in the control valve unit 8 is a fixed flow type, the quantity Q.sub.1 of the hydraulic fluid to be supplied to the driving control system circuit 18 is constant irrespective of loading/unloading operations. The discharge quantity Q.sub.0 of the hydraulic pump 4 is set to a value obtained by adding the quantity Q.sub.1 of the hydraulic fluid to be supplied to the driving control system circuit 18 and the maximum quantity Q.sub.2 max of the quantities Q.sub.2 L, Q.sub.2 T of the hydraulic fluid to be supplied to the lift cylinder 14 or the tilt cylinder 16 in case of loading/unloading operations. Accordingly, as schematically shown in FIG. 2, when the operating distances of the loading and unloading control levers 10 and 12 are short and the quantities Q.sub.2 L, Q.sub.2 T of the hydraulic fluid necessary for the loading/unloading operations are small, the excessive quantity of the hydraulic fluid not used for the loading/unloading operations is returned from the control valve unit 8 to the oil tank 6 through a drain tube 26. Particularly when the loading/unloading operations are not executed at all, the total quantity Q.sub.2 max of hydraulic fluid is returned as excessive quantity to the oil tank. The energy loss of the excessive quantity of hydraulic fluid introduces the problems of a rise in hydraulic fluid temperature and increased fuel consumption.